Electrical measurement systems and techniques have long been used to characterize the properties of bulk materials, for example, resistivity, permittivity and permeability. These techniques have been adapted to measure characteristics of surfaces and thin films and have been combined with optical techniques for measuring further properties such as semiconductor type and concentrations and chemical bonding. Attempts to apply these electrical and optical techniques to the fine surface structures developed in semiconductor integrated circuits (ICs) have been stymied by the small scale of modern IC features, typically well below 100 nm, such that most measurement probes and beams average over the different features of the IC.
Atomic force microscopy has been developed to profile the topography of a specimen with a resolution of 10 nm and less. One type of atomic force microscope (AFM) includes a mechanical probe with a tip positioned at the end of a flexible cantilever. The tip is tapered to have an apex having a diameter of, for example, less than 50 or 100 nm though 5 nm is currently achievable. The sharp tip is typically realized through anisotropic etching of crystalline silicon to form sharp pyramidal tips with dimensions of a few silicon crystalline spacings although other tapered shapes such as conical can be formed and including different materials. Through atomic interactions between the tip and specimen sufficient to affect the cantilever flexing, the probe tip can be made to hover a small fixed distance above the specimen as the tip is scanned over the specimen. Thereby, the specimen surface can be profiled by such a mechanical AFM with vertical and horizontal resolutions on the order of nanometers.
Commercial products are available in which an infrared beam irradiates the sample adjacent the tip of a mechanical AFM. In one product from Anasys, we believe the infrared beam causes the sample to thermally expand, which expansion is measured by the mechanical AFM. Thereby, the infrared absorption can be measured. In another product from Molecular Vista, we believe the infrared beam affects the atomic interaction between the sample and the AFM tip as the infrared is absorbed in the sample.
As described by Lai et al. in U.S. Pat. No. 8,266,718, incorporated herein by reference, atomic force microscopy has been combined with microwave measurement techniques to incorporate a microwave probe into the AFM cantilever tip. A conventional AFM system automatically scans the microwave tip closely adjacent a sample surface to electrically characterize small areas of the sample and thus image the electrical characteristics of the scanned surface. Li et al. describe an improved microwave probe tip in U.S. Pat. No. 8,307,461. Their microwave probe tips tend to be relatively blunt, fragile, and complex and are not beneficially used in a mechanical AFM designed for topographic profiling. PrimeNano, Inc. of Santa Clara, Calif. markets the ScanWave™ module for AFMs to provide high-resolution imaging of permittivity and conductivity of materials at the nanoscale.
Infrared absorption is often used to characterize materials, particularly semiconductors. Infrared absorption may be measured by comparing the intensity of infrared radiation incident on a sample and that of the radiation exiting the sample.
Carrier lifetime is an important quantity in semiconductor materials, for example, minority carrier lifetimes should be maximized in photovoltaic devices. Carrier lifetimes can be measured by placing a semiconducting sample into a macroscopic microwave cavity. The properties of the resonant cavity will vary according to the conductivity of the sample. If the sample is pulsed with laser light, the conductivity will decay on a time scale proportional to the carrier lifetime. As a result, the carrier lifetime can be extracted from the time dependent changes in the properties of the resonant cavity.
However, the spatial resolution of optical techniques is usually limited by the size of the beam illuminating the sample, typically on the order of microns or larger. It is desired to measure infrared absorption and carrier lifetimes on a much finer resolution.